A review of epidural and non-epidural contrast flow patterns during fluoroscopic and CT-guided epidural steroid injections

Abstract

Epidural steroid injections are commonly performed using fluoroscopic or CT guidance. With both modalities, the injection of contrast material is necessary before steroid administration to ensure adequate epidural flow and exclude non-epidural flow. While fluoroscopic guidance is conventional, CT is utilized at some centers and can be particularly helpful in the setting of challenging or postoperative anatomy. It is important for proceduralists to be adept at evaluating contrast media flow patterns under both modalities. The goal of this review article is to describe and provide examples of epidural and non-epidural flow patterns on both conventional fluoroscopy and CT. Specific non-epidural patterns discussed include intrathecal flow, intradural/subdural flow, vascular uptake, flow into the retrodural space of Okada, inadvertent facet joint flow, and intradiscal flow.

Introduction

Epidural steroid injections (ESIs) are well-established as a safe and efficacious treatment for patients with back pain and/or radicular symptoms. ¹ These injections are most commonly performed under fluoroscopic or CT guidance, which permits precise needle positioning, as well as visualization of contrast media flow prior to medication injection. ESI can be performed via a transforaminal approach, during which the target is the intervertebral foramen inferior, superior, or posterior to the nerve, or an interlaminar approach, during which the target is the dorsal epidural space. The complex anatomy around these targets (Figure 1) allows for the possibility of non-epidural needle positioning, which could lead to suboptimal or adverse outcomes.^2,3 It is therefore critical for the physician to be familiar with the appearance of epidural and non-epidural contrast flow patterns under conventional fluoroscopy and CT for both techniques to ensure appropriate flow of injectate.

Figure 1.

It demonstrates the complex anatomy of the spinal canal, which allows for variable contrast flow patterns during image-guided epidural steroid injection.

While previous descriptions and examples of flow patterns under fluoroscopy during ESIs exist, they are largely spread throughout individual case reports, textbooks, and/or injection manuals, which may be difficult or costly to access. Additionally, where comprehensive reviews do exist, there is a relative paucity of reference material available for CT-guided ESIs. The purpose of this review article is to create a comprehensive and accessible resource which contains examples of epidural and non-epidural contrast flow patterns on both fluoroscopy and CT. Non-epidural flow patterns to be discussed include intrathecal flow, subdural/intradural injections, vascular (arterial or venous) uptake, flow into the retrodural space of Okada, facet joint flow, and intradiscal flow. The examples found in this article will help physicians recognize and avoid potentially hazardous non-epidural injection patterns.

Methods

IRB approval was obtained for this retrospective article, and the requirement to obtain informed patient consent was waived. HIPPA guidelines were followed.

A review of available published literature was performed using PubMed. No date limit was set for the search. Search terms utilized were “epidural injection” plus the flow pattern of interest, for example: “intrathecal, subdural/intradural, vascular, retrodural space of Okada, facet joint, intradiscal.”

Additionally, case examples for each flow pattern were identified by a retrospective search of a database at our institution which includes the reports from all image-guided epidural steroid injections. The same terms were utilized for the search as above. Again, no date limits were set. All cases identified were reviewed by at least one of the authors to confirm the stated injection pattern.

Epidural contrast flow

It is necessary to obtain epidural flow for safe and effective ESIs. The central epidural space surrounds the thecal sac and contains varying amounts of fat within its ventral, lateral, and dorsal components. ⁴ The peripheral epidural space does not have strictly defined boundaries but is generally considered to include the intervertebral foramen and fat immediately adjacent to the foramen.

Interlaminar injections, during which access to the epidural space is gained through the interlaminar space, provide a greater proportion of central epidural flow. However, this most often involves the dorsal epidural space, with the presumption that some medication flows to the ventral epidural space. Interlaminar injections can be performed with a midline or paramidline approach, the latter of which may allow for right or left predominant flow in patients with unilateral symptoms. The paramidline approach has been shown to provide greater therapeutic benefit overall, presumably due to greater flow of medication to the ventral epidural space. ⁵ Both interlaminar and transforaminal ESIs have been shown to have comparable safety profiles.^6,7

For the interlaminar approach, initial contrast injection should disperse away from the needle tip in all directions on AP view (Figure 2). It will form a smooth but variable and irregular shape and may be asymmetric in its distribution. Characteristic vacuolization may be seen due to the presence of fat globules within the epidural space. Lateral view demonstrates a linear distribution of contrast, typically predominantly located within the dorsal epidural space. Contrast will be persistent and ultimately may outline the nerve root sleeves. ⁸ On CT, contrast media will flow away from the needle tip into the dorsal epidural space. With additional injection, it will continue to expand within the epidural space and may extend ventrally and outline the nerve root sleeves (Figure 3). Borders will be well-defined.

Figure 2.

Epidural contrast flow during fluoroscopic interlaminar injection at L3-L4. AP (a) and lateral (b) images demonstrate final needle tip position. Injected contrast media is heterogeneous in density on AP view with characteristic vacuoles ((c), dashed arrows). On postcontrast lateral view (d), contrast is homogeneous and well-defined, located within the dorsal epidural space (arrows).

Figure 3.

Epidural contrast flow during a right paramedian L5-S1 interlaminar injection. Initial injection of contrast media (b) is well-defined and homogeneous, confined to the dorsal epidural space (arrow). Wash out image (c) shows that contrast now extends into the ventral epidural space (dashed arrow).

During transforaminal ESI, contrast material is injected through a needle within the intervertebral foramen, either inferior, superior, or behind to the dorsal root ganglion (DRG) or exiting nerve (Figure 4). Depending on the precise position of the needle tip, as well as various complex anatomic factors such as the degree of neural foraminal stenosis, contrast media may flow into the central and/or peripheral epidural space. In general, transforaminal injections provide greater coverage of the DRG and lateral epidural space.

Figure 4.

Epidural flow pattern during fluoroscopically guided left L5 TFESI. Lateral (a) and AP (b) images demonstrate intraneural positioning of the needle tip prior to contrast media injection (solid arrow). Contrast media injection filled the intervertebral foramen along the nerve root sleeve. Continued injection and washout image showed excellent central and peripheral epidural flow ((c–e), dashed arrows), with a linear configuration and uniform distribution on lateral view. Contrast media density is heterogeneous on the AP views.

With a transforaminal approach, contrast material should initially outline the spinal nerve within the foramen. With further injection, contrast media should mostly extend centrally along the course of the nerve root sleeve and contact the medial aspect of the ipsilateral pedicle (Figures 4 and 5). There will often also be some peripheral flow along the exiting spinal nerve, although this should not be the dominant portion of the injectate. If mostly peripheral contrast flow is demonstrated, more medial positioning of the needle tip should be attempted if this can be done safely. Centrally, contrast will have a linear configuration in the epidural space as it outlines the thecal sac on lateral view. Contrast may be located within either the dorsal or ventral epidural space. There may be subtle motion of the contrast media due to dural pulsations. ⁸ On CT, contrast media will initially be seen around the needle tip within the foramen. With further injection, contrast will extend centrally and may flow into the ventral and/or dorsal epidural space (Figures 5 and 6). As with fluoroscopy, central epidural contrast will be relatively uniform with well-delineated margins and will persist after injection is stopped. Peripheral epidural contrast flow will have less well-defined margins.

Figure 5.

Robust ventral epidural flow pattern during fluoroscopically guided right L3 transforaminal epidural steroid injection. AP (a) and lateral (b) images demonstrate final infraneural needle tip positioning. Injected iodinated contrast media (c, d) demonstrated robust central flow with cranial and caudal flow (arrows). AP image demonstrates a small vacuole characteristic of the epidural space (arrowhead). On lateral view, contrast is closely approximated to the posterior aspect of the vertebral bodies and has a sharp dorsal margin also consistent with epidural flow. Subsequent axial (e) and sagittal (f) CT images confirm central epidural flow which extended across several levels and is mostly ventral (arrows). There was also some contrast flow along exiting nerve roots bilaterally ((e), open arrows).

Figure 6.

Epidural contrast flow during CT-guided right L4 transforaminal epidural steroid injection. Noncontrast image (a) demonstrates final needle tip position. Contrast media filled the intervertebral foramen ((b), solid arrow) and flowed into the central epidural space ((c), dashed arrow).

Intrathecal (subarachnoid) contrast flow

Intrathecal (subarachnoid) injection is an undesirable flow pattern that can be inadvertently obtained during either interlaminar or transforaminal ESI. Intrathecal injections are more commonly observed during interlaminar injections if the needle is advanced too far anteriorly. ⁶ This occurred in 0.8% of lumbar ESIs in a series of nearly 1500 cases. ⁹ Although it should not occur with proper technique, a transforaminal needle can be inadvertently placed intrathecally if a substantial lateral to medial trajectory is used, allowing the needle to pierce the lateral aspect of the dura (Figure 7). The reported incidence of intrathecal access during lumbar TFESI has ranged from 0% to 3.1% in a series of 257 infraneural TFESIs. ¹⁰

Figure 7.

Intrathecal uptake in two different patients during left L5 (a, b) and right L4 (c, d) transforaminal injections. In the first case, AP view (a) after contrast injection (a) demonstrates adequate epidural flow, as well as separate contrast flow projecting over the central spinal canal ((a), arrows). Lateral view (b) demonstrates layering contrast with a gradient appearance ((b), arrows), comparable with intrathecal flow. In the second case, AP view (c) also demonstrates some epidural flow, as well as separate contrast projecting over the central canal with a “waisted” appearance ((c), arrows). Lateral view (c) similarly shows a gradient or “fluid–fluid level” appearance ((d), arrows).

Several characteristic features can be used to identify intrathecal uptake and distinguish it from epidural flow. On the AP projection, intrathecal contrast media demonstrates uniform distribution into an hourglass or “waisted” shape due to slight narrowing of the thecal sac at each spinal level, with even distribution across the midline (Figures 7 and 8).^2,3 Contrast freely mixes with the cerebrospinal fluid. ⁸ This forms the characteristic gradient appearance on lateral view, where intrathecal contrast demonstrates a fluid–fluid level with ill-defined margin dorsally and increasingly dense contrast media up to a sharp ventral margin. In contrast, ventral epidural contrast media has a sharply defined dorsal margin against the dura. Finally, intrathecal contrast washes away due to superior or inferior flow, particularly if the patient is tilted in the caudocranial axis. In some instances, preferential ventral epidural flow can mimic intrathecal flow (Figure 5). If enough contrast media is injected, the nerve root sleeves will be filled. If intrathecal uptake is observed during ESI, it is reasonable to abort the procedure or attempt injecting at a different spinal level.

Figure 8.

Intrathecal uptake during an interlaminar epidural steroid injection (a-d). Lateral view prior to contrast injection (a) shows needle positioning at L5-S1. Lateral views after contrast injection (b, c) demonstrate intrathecal uptake with a typical gradient appearance (arrows). The needle was removed and placed at the level above, ultimately achieving epidural flow (d, open arrowhead), with persistent intrathecal contrast from the prior injection (d, dashed arrow). In a separate patient undergoing a CT-guided blood patch procedure 3 days after lumbar puncture (e-f), precontrast image shows adequate needle placement in the dorsal epidural space (e, arrow). After contrast injection, a small amount of contrast was seen leaking into the thecal sac through the known defect (f, arrowhead) in addition to epidural contrast (f, arrow). The injection was performed at the level above instead.

Combined epidural/intrathecal

It is possible to obtain a mixture of epidural and intrathecal contrast flow. For example, in the setting of inadvertent intrathecal access during interlaminar ESI, injectate may leak into the thecal sac if the needle is only simply withdrawn before injection due to a persistent dural hole (Figure 8). In such a case, the contrast media flow pattern would show features of both patterns described above. Careful review of live imaging is necessary to identify any features which suggest non-epidural flow.

Subdural/intradural contrast flow

The subdural or intradural space is a potential space between the layers of the dura. Literature regarding inadvertent injection into this space has termed it as either the subdural or intradural space, but the latter is now thought to be a more accurate description. Prior studies depicting electron microscopy of the dura suggest that most injections that radiographically appear intradural/subdural are in fact likely in one of the deep layers of the dura. ¹¹ Intradural injections most commonly occur during myelography, presumably when the needle tip is partially intradural. However, intradural injections may also be rarely encountered during ESI, more commonly when an interlaminar approach is used. ¹² Rarely, an intradural injection can happen during attempted transforaminal injection. ¹³

Intradural injection may have a variable appearance based on the level of resistance to cleavage by contrast media that the space exerts. ⁸ With initial injection in the AP projection, intradural contrast will form a thin film and may be difficult to appreciate. When viewed laterally, however, the contrast will form a smooth linear opacity parallel to the thecal sac. Injected contrast media may migrate superiorly, inferiorly, and circumferentially around the margins of the thecal sac, and the appearance can vary depending on exactly which dural layer the contrast distributes in. Contrast flow in the craniocaudal direction can be encountered if injectate is within the deep layers of the dura, and this can theoretically lead to respiratory compromise if anesthetic is injected and travels superiorly. ¹¹ When the potential intradural space resists contrast injection, this will result in a more globular, sharply marginated contrast opacification near the site of injection (Figure 9). This pattern of flow typically expands radially, rather than longitudinally, and may cause symptoms due to mass effect.^2,3 Furthermore, injections may be partially subdural and partially intrathecal, which can be confounding if only one of these two flow patterns is recognized. Ideally, if intradural flow is recognized, it is advisable to withdraw or replace the needle and attempt to achieve epidural flow.

Figure 9.

Mixed intrathecal and intradural contrast injection during attempted myelography. AP fluoroscopic image (a) after contrast injection demonstrates intrathecal ((a), arrows) contrast outlining the cauda equina nerve roots, as well as subdural contrast ((a), dashed arrows), which has a more globular morphology. CT scan of the lumbar spine obtained 30 min afterwards (b) shows intradural contrast ((b), dashed arrows) displacing the thecal sac. Fluoroscopic image (c) and sagittal CT (d) in a different patient undergoing lumbar myelography demonstrates multiple areas of globular, intradural contrast ((c, d), dashed arrows) admixed with intrathecal contrast ((c, d), solid arrows).

Vascular contrast flow

Vascular uptake can be arterial or venous; however, the vast majority of unintentional vascular injections are venous. In general, vascular uptake is encountered most commonly during transforaminal ESIs (Figures 10 and 11), but can be seen with interlaminar ESIs as well (Figure 12). Vascular flow has been reported to occur in 0.4–11.2% of lumbosacral transforaminal ESIs, with the highest rate of vascular flow seen at S1.^10,14–16 Rates of vascular access is reported to be around 2% in interlaminar injections, with higher rates during a paramedian approach.^17,18 Of note, one study showed that there was no difference in the rate of vascular access between 22- and 25-guage needles.^17,19

Figure 10.

Angiographic appearance of the anterior spinal artery (a, b) and arterial flow during a transforaminal injection (c–e). Lateral (a) and AP (b) images during an angiogram demonstrate contrast flow within radiculomedullary arteries at C5 and C6. Lateral image shows the characteristic “hairpin turn” at its anastomosis with the anterior spinal artery. Lateral ((c) precontrast, (d) postcontrast) and AP ((e) postcontrast) fluoroscopic images in a patient undergoing a right L5 transforaminal injection show opacification of thin vessels in the foramen and coursing anteriorly and laterally ((d, e), arrows). These washed away rapidly and were thought to represent medullary arterial branches. Therefore, the procedure was aborted and the patient was brought back on a different day.

Figure 11.

Venous flow during CT-guided transforaminal injection. Axial images during a CT-guided L4 transforaminal epidural steroid injection demonstrate venous flow during contrast injection ((a, b), arrows). Appearance is the characteristic well-defined, tubular shape in the soft tissues posterior and lateral to the targeted foramen. Peripheral epidural flow was also demonstrated ((c), dashed arrow).

Figure 12.

Venous flow during an L4-5 interlaminar epidural steroid injection. Precontrast AP (a) and lateral (b) images demonstrate initial needle position. Subsequent injection of iodinated contrast (c, d) demonstrates venous flow (arrows), extending peripherally into the paraspinal soft tissues.

Each nerve root is accompanied by an artery, which most commonly supplies the neural elements of the foramen and is termed a radicular artery. ²⁰ Those which also supply the spinal cord are radiculomedullary arteries, the largest and typically most caudal of which is the artery of Adamkiewicz (AKA). ²⁰ In general, arteries within the intervertebral foramen are most commonly located in the upper portion of the foramen, with AKA most frequently on the left between T8 and L1.^20,21 Arterial injections are similarly undesirable, but they are also feared due to the potential of injection into and occlusion of the AKA or another major branch supplying the anterior spinal artery, potentially resulting in cord infarction. ²² Most case reports of paralysis thought to be caused by ESI occurred in the setting of particulate steroid injection with some concern for arterial flow during the procedure, leading to the shift away from particulate steroid use for ESI. ²³

Vascular flow demonstrates uniform distribution of contrast media within a tubular structure with well-defined margins, forming a serpiginous configuration as vessels branch (Figures 11 and 12). Upon stopping contrast injection, the contrast will clear from the area. ⁸ There will most commonly be a delayed wash out of contrast following the termination of injection, although arterial flow will clear more rapidly. ¹⁷ Flow within the AKA will similarly demonstrate uniform contrast within a well-defined tubular structure; however, the vessel will travel along the ventral nerve root to the ventral surface of the spinal cord where it makes the characteristic hairpin turn toward the anterior spinal artery (Figure 10). ²⁴ Blood vessels are typically identified near the needle tip and may also extend into the paraspinal soft tissues. ¹⁷

Vascular flow can usually be cleared by either advancing the needle through the vessel or placing the needle in a more superior or inferior location relative to the vessel. In rare cases where flow is felt to be arterial (based on rapidity/direction of contrast flow), it is typically necessary to abort the procedure or inject at a different level. ²⁵ Furthermore, since anterior spinal artery branches such as the AKA more commonly exist in the superior half of the intervertebral foramen, an infraneural approach can be used if the artery in encountered during a supraneural injection.^21,26

Injection into the space of Okada

The retrodural space of Okada is a potential space located immediately dorsal to the ligamentum flavum. ²⁷ It is a complex space that usually communicates with the adjacent facet joints and interspinous ligament. It may also communicate with adjacent defects of the pars interarticularis, if present. It is a clinically important anatomic space that can act as a conduit for spread of fluid or inflammation. ²⁸ The space of Okada is most commonly accessed during routine facet joint injections, which is typically of no clinical consequence. However, this space may be inadvertently accessed during both transforaminal and interlaminar ESI. In the case of transforaminal injections, this usually occurs due to accidental needle placement into the capsule of an adjacent facet joint, which frequently extends into the intervertebral foramen (Figure 13).^29–31 During interlaminar ESI, the space of Okada may be accessed if injection is attempted when the needle tip is immediately dorsal to the ligamentum flavum.

Figure 13.

Uptake into the facet joint and retrodural space of Okada during right L4 transforaminal injection. Lateral (a) and frontal (b) precontrast images show the needle tip projecting over the right L4 foramen ((a, b) arrows). After injection of a small volume of contrast, there was opacification of the right L4-L5 facet joint ((c), solid arrows) and the central and peripheral epidural space ((c), dashed arrows) on the lateral projection. Continued injection of contrast demonstrated partial central and peripheral epidural flow ((d), dashed arrows), as well as flow from the right L4-L5 facet into the retrodural space of Okada, crossing midline from right to left ((d), solid arrows).

Contrast flow patterns within the space of Okada on both fluoroscopy and CT can mimic dorsal epidural flow. ³² Therefore, it is important to distinguish these flow patterns. Contrast media within the space of Okada will be located immediately dorsal to the ligamentum flavum. On the AP projection during fluoroscopy, contrast will flow centrally away from the needle tip and across the midline. Contrast will appear relatively uniform but irregular in shape. There may be communication to the unilateral or contralateral facet joint. On CT, contrast media will be uniform and well-marginated. Upon further injection of contrast, there may be communication to the interspinous region. ³¹

Facet joint flow

The facet joints can be accessed during both transforaminal and interlaminar ESI. In the former case, this typically occurs when the needle tip is placed relatively posteriorly in the intervertebral foramen such that a redundant portion of the superior recess of the facet joint capsule is accessed (Figure 14). Incidence of facet joint flow has been reported as 0.61% with transforaminal and 0–3.6% with interlaminar approach.^9,33,34 Initial injection of contrast media on AP view may mimic central flow. As more contrast is injected, however, the facet joint will become distended, forming a well-circumscribed, uniform, vertically oriented collection of contrast which is lateral to the epidural space.^2,3 The characteristic facet joint arthrogram may be more easily identified on an oblique or lateral projection. This can usually be corrected by minimally advancing the needle more anteriorly. During interlaminar ESI, facet joint access usually occurs if the retrodural space of Okada is inadvertently encountered. In this situation, contrast within the space of Okada would also be visualized as described above. This can similarly be corrected by advancing the needle through the ligamentum flavum into the epidural space.

Figure 14.

Uptake into the facet joint during right L5 transforaminal injection. Initial contrast injection during right L5 TFE in the lateral (a) and AP (b) projections initially was in an unclear location. However, with further injection of contrast, there was clear distention of the ipsilateral facet joint, characterized by vertical distribution and filling of the superior and inferior recesses on lateral view ((c), arrows). Advancement of the needle resulted in epidural flow in a characteristic distribution on AP view ((d), dashed arrow); however, the previously injected facet joint contrast persists ((d), arrow).

Intradiscal contrast flow

Intradiscal injection has been performed for decades for both diagnostic and therapeutic purposes, mainly in patients with disc degeneration. ³⁵ Accessing the intervertebral disc is also important for administration of various therapeutics thought to elicit disc regeneration. ³⁶ Inadvertent injection into the intervertebral disc is most likely to occur during transforaminal ESI, especially if using an infraneural approach. In patients without significant posterior disc bulge or herniation, the intervertebral disc may be accessed if the needle is placed too far ventrally. In patients with disc bulges or protrusions/extrusions, the disc can be more easily accessed even with appropriate placement within the neural foramen. The reported rate of intradiscal contrast varies widely, ranging from 0.08% in a large series, 3% in a series of 100 injections, and in 4.7% of cases in a report of 257 infraneural TFESIs.^9,10,15 Review of cross-sectional imaging prior to ESIs will alert the proceduralist to the presence of disc material within the neural foramen and allow for alternate needle positioning.

Discal contrast media flow typical conforms to the nucleus pulposus of the intervertebral disc and is easily recognized (Figure 15). Intradiscal contrast has a linear, horizontal distribution within the disc space on both AP and lateral views. Usually, minimally retracting the needle after discal flow is seen will result in epidural contrast flow, allowing for a therapeutic injection. Nonetheless, efforts should be made to avoid penetrating the intervertebral disc, which is avascular and particularly susceptible to iatrogenic infection. ³⁷

Figure 15.

Intradiscal flow during left L5 transforaminal injection. Axial (a) and sagittal (b) T1 W MR images demonstrate a disc bulge extending into the inferior left L5 neural foramen ((a, b), arrows). Lateral image during transforaminal epidural steroid injection (c) shows the needle tip projecting over the mid-neural foramen ((c), arrow). AP image obtained after contrast injection (d) shows a combination of epidural flow ((D), solid arrows) and a small amount of intradiscal flow ((d), dashed arrow). The needle was slightly retracted, and final flow was entirely epidural.

Conclusion

Knowledge of contrast media flow patterns is critical to safely and effectively perform ESIs, both under fluoroscopy and CT. Here, we have presented examples of epidural and non-epidural contrast flow on both modalities, focusing on various key features needed to correctly determine the location of injectate. Specifically, we have described epidural, intrathecal, intradural/subdural, vascular, space of Okada, facet joint flow, and intradiscal flow. For reference, these flow patterns are summarized in Table 1. While previous literature has mainly focused on fluoroscopically guided injections, the availability and use of CT is becoming more prevalent. It is becoming increasingly important for practitioners to be familiar with flow patterns in both modalities.

The authors declare that they have no conflict of interest. All authors contributed to writing and editing the manuscript.

No funding was received for this study. All data used remain available within our institutional database.

Table 1.

Summary of potential flow patterns during epidural steroid injection.

Flow Pattern	Shape	Margins	Density	Persistence
Epidural	AP: irregular, lobulated Lateral: linear	Smooth	AP: may be heterogeneous Lateral: uniform	Yes
Intrathecal (subarachnoid)	AP: hourglass	Lateral: gradient dorsal, sharp ventral	Uniform	Free flowing into CSF
Intradural/subdural	Linear streaks along the surfaces of thecal sac that are tangential to beam. May be linear or globular.	Smooth	Uniform	Yes
Venous	Narrow linear bands, serpiginous	Smooth	Uniform	No
Arterial	Narrow linear bands	Smooth	Uniform	No
Space of Okada	Irregular shape, crosses midline	Well-defined, better appreciated on CT	Heterogeneous	Yes
Facet Joint	Confined to facet joint, arthrogram	Smooth	Uniform	Yes
Intradiscal	Linear, horizontally oriented	Smooth	Uniform	Yes